Every major key contains three principle chords:
The most common sequence of these 3 chords is from tonic to sub-dominant to dominant 7th and back to tonic. In the key of C major, C I is the tonic, F IV is the sub-dominant and G7 V7 is the dominant 7th. Thus the basic chord sequence is: C - F - G7 - C, illustrated below:
You will notice that the F chord requires the first finger to "bar" across two strings in order to hold down both the C and F notes. This will be quite difficult at first, but can be made easier by tilting your first finger slightly to its left side when placing the bar. (See photo. 33).
Remember the L/H thumb must remain under the first finger (See photo. 34).